Work in progress is designed to study the influence of external pH variation on acetylcholine-induced currents recorded from snail neurons under conditions in which the currents are carried exclusively by sodium ions. Acetylcholine-induced currents are recorded under voltage clamp conditions and acetylcholine is applied electrophoretically to the neuron surface. The rate of current rise is measured at external pH values in the range 4.5 to 10. It is expected that the results will add some information concerning the site of action of hydrogen ions within the cholinergic receptor mechanism. Potential sites under consideration are: the charged groups on the receptor surface, the protein component of the receptor and the fixed membrane charges that regulate ion permeation through the acetylcholine-sensitive membrane.